Let's take an example. On most items the roll for a stat affix is 1-100, but for some stats on some items it is 1-200. You can get a double stat affix (say, Dex/Vit) where both range from 1-100.
- Compare three pieces of gear that are 150 dex, 75 vit, and 60 all resist.
- Gloves - these would not be valuable. Gloves get a 1-200 dex roll, people want attack speed, crit chance, and/or crit damage on there gloves, and lots of gloves have been made chasing these stats. Worth maybe 10k tops.
- Boots - also not valuable. Boots also get a 1-200 dex roll, and people want movement speed on their boots. 10k tops again.
- Shoulders - Jackpot. Dex only goes 1-100 on shoulders, so you would have needed to get both a Dex and Dex/Vit affix, making them rarer. There aren't competing dps stats people are chasing on shoulders, either. Add in life % or another resist and the value shoots up even higher, but even with the initial stats these worth over 2 million gold.
Once you know this you can make money on the AH. Even if you don't want to camp the AH, you can still find bargains as you price out the gear you find. Check the current prices of that blue 18% magic find ring you found (usually around half a million) and see one for 80k with a 1d12h buyout? Don't hesitate, grab it immediately before someone else does and relist it, you've made over a quarter of a million gold in 15 seconds work.
As for advice on how to grind out loot to sell in the AH, you want to find a route that works for you to first build up NV stacks and then hit elite packs quickly. There's been quite a bit of discussion here on routes if you search. Be efficient in what you pick up. Find out what works best for you in value- I pick up any 61+ rares (and all jewelry), and generally skip all blues that I can't sell on the AH. I have the names of items/levels memorized and some important affixes so I can quickly evaluate what's worth picking up (rare Sacred Shield, glittering ring, etc) and what's not (rare Hoplon, glittering balor pauldons.)